Heat grape juice and water in the top of a double boiler over direct heat.
Combine cornstarch, salt, and sugar, and mix thoroughly.
Gradually add the grape-juice mixture, while stirring constantly.
Return to double boiler, and cook over boiling water, stirring constantly, until smooth and thickened.
Then cook, covered, 15 min longer, stirring occasionally.
Add butter, and stir until melted.
Cool, add lemon juice, and stir until blended.
Use as a filling for Grape Layer Cake, 4 tablesp all-purpose flour may be substituted for the cornstarch, in which case slowly stir grape juice and water, into combined flour, salt, and sugar, then cook.